Considering how many android devices there are, which are the most popular ones I should test my game on?
Probably some on some low-end one (like samsung galaxy ace, galaxy mini, etc.) and high-end one with high resolution (like nexus 4/5, samsung galaxy s3/s4/note2/note3, htc one, sony xperia z) and maybe additional on some tablet.
So in general, the 2 things you should care about is overall performance and screen size/resolution.
Thanks for the feedback!